Vegetable oils find a wide variety of uses in consumer products including use as food additives, lubricants, solvents and coating additives. Most applications require refined vegetable oil that is essentially colorless to light yellow and free from insoluble material. Generally, vegetable oil is obtained by dry extruding vegetables, for example, vegetable seeds, in a friction extruder-usually at a temperature of greater than 130.degree. C. to provide a semi-fluid extrudate. Then the semi-fluid extrudate is processed in a screw press to yield a crude vegetable oil and a solid or semi-solid vegetable meal, which is separated from the crude vegetable oil. Despite this initial separation, the crude vegetable oil still contains a variety of suspended insoluble solids. The suspended insoluble solids include semi-solid or gummy material that can be particularly difficult to remove by typical mechanical separation means. The insoluble solids do not readily precipitate from the crude oil. Attempts to filter out the insoluble solids present problems because the semi-solid and/or gummy material clogs the filter medium.
In an effort to address these problem, in the past crude oil has been refined using a solvent extraction process. In a solvent extraction process the crude oil is extracted with an organic solvent, for example, a mixture of hexanes. The organic solvent absorbs greater than 90% of the seed oils. The extract can be filtered, if necessary, to remove any particulate and/or insoluble matter. After filtering, the solvent is removed, typically by evaporation, to provide a partially refined vegetable oil, which can be used in many commercial products or further processed if desired.
While solvent extraction provides a process for refining vegetable oil, this process presents some significant problems. Along with the insoluble material, many of the beneficial, natural nutrients such as antioxidants and vitamins are removed from the vegetable oil. Both the extracted oil and solid material inevitably contain residual hexanes. Hexanes are toxic to humans and other animals and must be removed before the resulting oil and solid material are suitable for consumption by humans and other animals. While the hexanes are recycled for reuse, inevitably a significant amount of hexanes escape into the atmosphere and contribute to environmental pollution. In addition, hexane is an extremely flammable solvent and, therefore, presents a significant risk to fire and explosions. It is desirable to provide an alternative process for purifying vegetable oil.
Thus, there is a need to provide a process for purifying vegetable oils that avoids the problems described above. The present invention addresses this need in a novel and nonobvious way.